1. Field of the Invention
The present invention relates to a method of fabricating an electronic circuit device in which an electronic part is mounted in a cavity formed in a package main body via a die bonding material and a conductive paste for wiring, and the cavity is sealed in an airtight manner via a cap and a sealing material.
2. Description of Related Art
Conventional semiconductor acceleration sensors used, for example, in an air bag system or ABS (Antislip Braking System) typically include a metal can package of a pin inserting type. However, in a semiconductor acceleration sensor using a can package, the number of parts is often large, the structure is complicated and the total configuration size is magnified. Further, since the sensor is of a pin inserting type, the process of mounting the sensor to a printed circuit board or the like is troublesome.
In recent years, semiconductor acceleration sensors that utilize a ceramic package of a surface mount type have been developed to resolve the above-described problem. That is, electronic parts of a semiconductor acceleration sensor chip, an IC (Integrated Circuit) chip for processing the sensor output and the like are die-bonded into a cavity formed in a ceramic package main body having terminals for surface mount. The cavity is then sealed in an airtight manner by a cap.
In this case, sealing by high temperature solder, sealing by resin, sealing by low melting point glass or the like is conceivable for sealing the cap. However, in carrying out sealing by high temperature solder, thick wall gold plating is needed for an electrode installed to the package. Therefore, the package itself per se becomes expensive. Further, in carrying out sealing by resin, it is difficult to maintain the airtight state of the sealed space over a long period of time since it has minimal humidity resistance. Therefore, conventionally, the sealing by glass having a low melting point has generally been adopted.
Further, depending on the state of mounting a semiconductor acceleration sensor chip, it may be required to alleviate stress at a die-bonded portion thereof. Therefore, a silicone group die bonding material having a low Young's modulus is used. Further, when wiring of electronic parts is needed, a silicone group silver paste capable of curing simultaneously with the silicone group die bonding material is used as a conductive paste for the wiring.
In carrying out detailed measurement, including transmission X-ray observation, of a semiconductor acceleration sensor which has been manufactured by being processed by a step of sealing a cap by glass having a low melting point as described above, it has been found that a bond width of a glass seal portion may be thin compared with a predicted width. Further, occurrences of a phenomenon in which a void is caused at the glass seal portion, and a phenomenon showing a state where a cap is inclined form a regular attaching state have been observed.
It seems that such phenomena occur due to gases generated from the silicone group die bonding material and the silicone group silver paste. That is, in the cap sealing step, a heat treatment is performed for elevating the temperature of the low melting point glass to the melting point temperature. It has been found that, in accordance with such a heat treatment, gases produced by thermal decomposition are generated from the silicone group die bonding material and the silver paste. The inner pressure of the cavity is also considerably increased which causes thinning of the bond width of the glass seal portion, voids and the like. It is conceivable that such phenomena are also present when using a die bonding material and a conductive paste other than those of the silicone group. At any rate, deterioration in quality of a finished product may be caused due to deterioration in the reliability of sealing, or mount failure caused by the inclination of a cap may result in mounting the finished product on the surface.